Photolithography is a common technique used for manufacturing microelectronic circuits and other microfeature devices. In a typical photolithographic process, a microfeature workpiece (e.g., a silicon wafer) is coated with a photosensitive material. Selected portions of the photosensitive material are then exposed to a radiation beam, while neighboring portions are protected from exposure to the radiation by a mask. As a result of being exposed to the radiation, the selected portions can become resistant to a photoresist etchant, or susceptible to the photoresist etchant. Accordingly, when the workpiece is exposed to the photoresist etchant, the photosensitive material is removed from either the exposed regions or the protected regions. The photosensitive material remaining on the workpiece can protect the workpiece material underneath during a subsequent process, such as an etching process for removing material from the workpiece. Using this technique, material can be selectively removed from some portions of the workpiece but not others, allowing the formation of structures (e.g., circuit elements and conductive lines and/or vias) in the workpiece.
FIG. 1 is a partially schematic illustration of a photolithographic system 10 configured in accordance with the prior art. The system 10 includes a workpiece support 11 that carries a microfeature workpiece 12 beneath a lens system 20. An upper surface 13 of the workpiece 12 is selectively exposed to radiation passing along a radiation path 42 through a series of lenses, including a terminal lens 21. The region between the terminal lens 21 and the workpiece surface 13 is filled with a liquid volume 22, forming an immersion lens system. The liquid in the liquid volume 22 can include water or another liquid having a relatively high index of refraction. Accordingly, the photolithographic system 10 can image smaller features on the workpiece 12 than would be the case if the region between the lens 21 and the workpiece surface 13 were filled with air.
In operation, the workpiece support 11 scans or steps the workpiece 12 relative to the lens 21 by moving sequentially along transverse axes A and B. As the workpiece support 11 moves, liquid is continuously supplied to the liquid volume 22 through one or more supply ports 23 and removed from the liquid volume 22 through one or more return ports 24. The roles of the supply ports 23 and the return ports 24 can be reversed when the motion of the workpiece support 11 reverses. In this manner, the liquid volume 22 can remain in a generally fixed location relative to the terminal lens 21 as the workpiece 12 moves relative to the terminal lens 21.
The terminal lens 21 can be supported at a selected distance away from the upper surface 13 of the workpiece 12 by an air bearing 30. Accordingly, the air bearing 30 can include a plurality of air bearing nozzles 31 through which air is injected downwardly toward the upper surface 13, forming an air cushion 32 between the lens system 20 and the workpiece upper surface 13. Vacuum ports 25 are positioned around the periphery of the liquid volume 22 to withdraw residual fluid that may remain on the workpiece upper surface 13 after it contacts the liquid volume 22. The vacuum ports 25 can also be used to secure a cover over the terminal lens 21 when the system 10 is not in use.
One drawback with the foregoing arrangement is that, despite the presence of the vacuum ports 25, residual liquid can remain on the workpiece upper surface 13 after the upper surface 13 passes beneath the liquid volume 22. Remaining water droplets can leave stains or other marks on the upper surface 13 after the droplets themselves evaporate. This has been the case even when highly purified water is used to form the liquid volume 22. These marks can interfere with downstream processes, reducing the efficiency and/or effectiveness of these processes and therefore the overall process of forming microelectronic features in the workpiece 12.